                           lessonTypes))
 
             affectedDaysSet
-            (reduce (fn [daySet augTTLessonInfo]
-                      (conj daySet (:day augTTLessonInfo)))
-                    #{}
-                    augTTLessonInfoSeq)]
+            (get-set-from-seq-of-maps-with-day-key augTTLessonInfoSeq)]
 
         ; Remove module from `ModulesSelected`
         (set! ModulesSelected (dissoc ModulesSelected moduleCode))

          destLessonGroup (data $helper LESSON-GROUP-CHANGE-KEY)

          affectedDaysSet
          (reduce (fn [daySet ttLessonInfo]
                    (conj daySet (:day ttLessonInfo)))
                  #{}
                  Lessons-Created-By-Draggable)]

      (doseq [augTTLessonInfo Lessons-Created-By-Draggable]
        (timetable-remove-lesson! (:day augTTLessonInfo)

      (if (not (nil? destLessonGroup))
          (let [augTTLessonInfoSeq (remove-lesson-group-html moduleCode
                                                             lessonType)
                affectedDaysSet    (get-set-from-seq-of-maps-with-day-key
                                     augTTLessonInfoSeq)]
            ; remove the current lesson group
            (doseq [augTTLessonInfo augTTLessonInfoSeq]
              (let [day          (:day augTTLessonInfo)
